1. Field of the Invention
This invention relates to a pipe coupling and seal More specifically, it relates to a pipe coupling which incorporates a spring member to control the timing of the gripping and sealing of opposite pipe ends to be connected and further reduces torque loss caused by stress relaxation of the pipes or sealing gaskets.
2. Description of the Art
The technology of sealing and gripping two plain-end pipes with a bolted coupling is known in the prior art. See for example U.S. Pat. Nos. 4,070,046; 4,606,565; and 4,569,542; all of which are currently being utilized for this purpose.
U.S. Pat. No. 4,070,046 discloses pipe coupling for both restraining and sealing the pipe ends. One major disadvantage of this design results from the timing as to when the gripping and sealing members are actuated Because the gripping members engage the pipe ends at very low torque, the follower member becomes locked onto the pipe end with this design. Accordingly, proper gasket compression can only be achieved by insuring that there is a gap between the pipe ends and insuring that at least one of the pipes is free to move axially into the coupling These requirements make this design unsuitable for use in certain applications where such pipe movement is not possible Another problem with this design is that gasket compression may be reduced when the coupled pipe is under an axial tension load because the gripping member which supports the gasket has a tendency to move away from the gasket
U.S. Pat. No. 4,569,542 discloses a device which provides partial control of the timing of the gripping and sealing functions of a pipe coupling by providing a gasket member having a rib thereon designed to offset the gasket from the gripping member when in a relaxed state Referring to FIG. 3 of U.S. Pat. No. 4,569,542, such coupling utilizes an extension 40 on the sealing member to create a space Y below the seal This space is utilized as a timing mechanism so that the sealing system is substantially activated prior to the gripping system. Bolt tightening during installation initially causes compression of the rib (and thus the gasket) prior to engaging the gripping member. Additional tightening locks the follower onto the pipe end, but presumably not until the gasket is sufficiently compressed.
The use of the gasket member in U.S. Pat. No. 4,569,542 to provide a desired sequence of the sealing and gripping functions of the coupling causes varying and unreliable results. First, the behavior of the compressibility of the gasket rib necessarily depends upon the hardness of the rubber, the temperature, and the physical dimensions of the rib, all of which are difficult to control. The result is that varying installations will have different degrees of sealing and gripping pressures. Secondly, after the installation of such a coupling, the extension of the gasket will be highly stressed and being rubber, will relax by some degree over time This relaxation is identified by a lowering of bolt torque values over time. Such a reduction of torque values will be greater where one or more of the pipes to be joined is plastic because plastic pipes will also suffer from stress relaxation over time Finally, with this design, it does not appear to be possible to provide a specific desired degree of gasket compression and gripping member load because the former is still dependant upon the later
Another partial solution to these problems have been proposed in U.S. Pat. No. 4,606,565 in which separate follower members are provided for the gripping and sealing members. With such a coupling, complete and repeatable gasket compression as well as complete and repeatable gripping engagement are possible regardless of installation conditions. While such a coupling solves the problems of gasket compression and gripping engagement, the design requires assembly of a number of pieces and proper tightening of twice as many bolts. Accordingly, U.S. Pat. No. 4,606,565 overcomes the limitations of the other designs, but, in so doing, increases the number of pieces and causes the installation of the device onto the pipe to be somewhat cumbersome and overly time consuming.
There remains, therefore, a need for an improved pipe coupling which provides a desired degree of sealing and gripping functions regardless of installation conditions There remains a need for such a coupling which is simple to install and manufacture. There still further remains a need for such a coupling which retains an appropriate seal over time